libxl: Make domain_shutdown fail if graceful not possible

libxl: Make domain_shutdown fail if graceful not possible

Currently "xl shutdown" (like "xm shutdown") is not capable of doing
the proper ACPI negotiation with an HVM no-pv-drivers guest which
would be necessary for a graceful shutdown.

Instead (following the ill-advised lead of "xm shutdown") it simply
shoots the guest in the head.

This patch changes the behaviour so that "xl shutdown" fails if the
domain cannot be shut down gracefully for this reason and suggests in
the error message using destroy instead.

Also, check whether the PV shutdown protocol is available before we
try to use it.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>